Determination of the Mean Density of the Earth. 219 
Although there was probably when I wrote already sufficient 
experimental evidence to show that induction took place across 
such a vacuum, yet I thought it worth while to make the 
experiment described in my letter, in which a platinum ball 
suspended in such a vacuum is shown to be strongly attracted 
towards a neighbouring electrified platinum sheath, precisely 
as if suspended in any other fluid dielectric, while a minute 
sp’rk was ceen at the moment when the two attracting bodies 
carie into contact. 
This tube and others that I experimented with exhibited 
the luminous effects described by Prof. Edlund when moved 
in the neighbourhood of an electrified body or influenced by the 
passage of currents in the neighbourhood; but 1 do not think 
that this luminosity is necessarily the proof of a current in the 
ordinary sense. It may be due to the increase or diminution 
of induction within the medium, some of the energy of the 
induction-tubes being absorbed in transit. 
It would be interesting to make the experiment of passing 
a stroug current through a stout wire surrounded by such a 
vacuum, and to observe whether the transit of energy across 
the vacuum ™* was attended by any luminous effect. If it were, 
then the vacuum might be regarded as a conductor in so far 
as the inductive energy entering is transformed into the lumi- 
nous form. If no effect were observed, we should infer that 
this transformation did not take place when a steady flow was 
reached, but only accompanied variations in the flow. 
I am your obedient servant, 
Montreux, February 14, 1885. A. M. WortTHINGTON. 
XXV. On the Application of the Pendulum to the Determi- 
nation of the Mean Density of the Earth. By Dr. J. 
Witsine, of Potsdam ft. 
» ae first experimental determination of the mean density 
of the Harth which Maskelyne and Hutton carried out 
depends on the observation of deflection of the plumb-line in 
the neighbourhood of Schehallien. It is limited by the uncer- 
tainty arising from our imperfect knowledge of the mass and 
density of the deflecting mountain, an error which also affects 
the results deduced by Carlini and Airy from oscillations of 
the pendulum. Accordingly the use of the pendulum has been 
abandoned in favour of the torsion-balance, as it is not sensi- 
* See Prof. Poynting’s paper “On the Transfer of Hnergy in the 
Electromagnetic Field,” Phil. Trans., Part IT. 1884. 
+ Translated from the Sttzwngsberichte der Berliner Akad. der Wissen- 
schaften, January 1885. 
Q 2 
